This invention relates to the field of monitoring the energy level of storage batteries and in particular to a device for temperature compensating such measurements. At this time, approximately twenty million dollars is spent per year for lithium batteries for portable communication equipment. This is expected to increase to fifty million dollars a year in a few years when most of the batteries for portable communication equipment are lithium. Therefore it is beneficial to provide a monitor device for conveniently and reliably determining the remaining charge level or energy level of lithium batteries by the equipment operator.
The lack of such an energy monitor device has been a major cost of waste due to the disposal of partly used batteries. Conversely, if there is no means for determining the charge level of the lithium batteries, it cannot be determined when the battery needs to be replaced. This creates a potentially hazardous situation for the operator at low end voltage of the battery. It also results in reduced safety and reliability since partially used batteries which are not disposed of may be exhausted at inopportune times. Thus it is desirable to develop a low cost, lithium battery energy monitor which will provide a visual indication of the energy remaining in the battery to a user of equipment powered by lithium batteries.
It is known in the art to monitor battery consumption using monitor circuits. U.S. Pat. No. 4,556,061 issued to Barrerars on Dec. 3, 1985, detects the voltage drop across a resistor in series with batteries supplying current to a cardiac pacer system. The voltage drop is amplified and applied to a voltage controlled oscillator which provides a pulse wave proportional to current flow through the resistor. A counter provides a cumulative count which represents battery consumption. External magnetic interrogation signals are applied to the pacer to retrieve the charge data. However, the system of Barrears does not provide any temperature compensation.
U.S. Pat. No. 3,895,284, issued to Schwiezer on Jul. 15, 1975, teaches the monitoring of starter batteries for motor vehicles. While the system of Schwiezer teaches the subject of temperature compensation, it does not compensate state of charge calculations on a continuous basis.
U.S. Pat. No. 4,323,849, issued to Smith discloses a coulometer wherein a signal representative of charge associated with a storage battery is applied to an integrator. The number of ampere-hours of available charge are displayed and some temperature compensation is performed. However, in the system of Smith temperature compensation is only performed with respect to thermal effects within the electronics rather than the temperature of the battery being monitored.
U.S. Pat. No. 3,906,329, issued to Bader, teaches a method for measuring the charge condition of galvanic energy sources and an apparatus for carrying out the method. However, the method and system of Bader measure charging current, or a magnetic property thereof, and weighs this with a factor dependent on the predetermined gassing voltage which changes with temperature. Furthermore, Bader pertains to secondary, rechargeable batteries. Thus Bader does not teach a method of measuring the relevant properties of primary lithium batteries, such as the remaining charge level in the battery.
U.S. Pat. No. 3,344,343, issued to John, teaches a retained capacity indicator using a mercury coulometer and thermister. Thus, the system of John does not provide a method of monitoring primary batteries so that the cumulative charge used is continually accounted for and compensated for temperature. Additionally, John does not at any time provide the percent of charge remaining which is available to the user.
Therefore, a monitor is needed to reduce indiscriminate replacement of partially used primary batteries during field operation and to permit reuse of partially discharged primary batteries. In order to achieve this a determination of battery performance parameters which feature very flat voltage discharge curves it is necessary to measure charge unit time, to add and store the results, and to activate appropriate visual indicators. A press-to-test status button is suggested to prevent the indicator from consuming power when not in use.
An object of this invention is a low cost lithium battery energy monitor that provides a visual indication of energy remaining.
A further object of this invention is to automatically process and integrate charge based upon activation of the load on the battery.
It is a further object of this device to provide a readout activated by a test switch.
In addition to monitoring current drain, the temperature must also be monitored since the rate of battery discharge varies significantly with temperature.